RPM dector redesign with motor with 4 wires?

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
My old Atari 810 is not working reliably. The problem seems to be with the board that controls the RPM of the drive motor. Supposedly it was a poor design from the start.

The rest of the drive works fine. It would be nice to redesign the whole analog board (which has been done btw but I want to build my own ;) ). But for starters, I am wondering in I might be able to redesign the RPM controller.

The drive motor is a 4 wire motor. My first question is what are the 4 wires? I assume one of them is a sensor?

Here is a block diagram of what they call the "analog board".

upload_2019-4-28_15-39-12.png
They say "motor current" supply which confuses me a bit. Isn't motor speed usually controlled by changing the voltage?

The drive is sort of working. I do have a schematic. Any tips on where to start collection of specifications doing measurement?

P.S. I still need to digest the service manual. Specs for drive motor may very well be in there.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Here is the relevant section of the schematic. I am trying to understand the i/o to that part of the circuit. Looks to me like Pin 9 off of the large connector is motor control? What would pin 11 be? And the connection on the far right? It is the +12V power supply on the right there. So would that connection simply be power?



upload_2019-4-28_16-9-3.png
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Stepper Motor.
Max.

The one that moves the head yes. But not the one that drives the disk. This is the motor that spins the disk. Look at the block diagram. I would think two of them have to be a sensor.

Also look at schematic just posted. J103 is going to motor in question.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
So I think I am right. Pin 11 is nothing more than an LED that is on the front panel. Pin 9 turns the motor on from a PIA. Does that make sense?


upload_2019-4-28_16-27-37.png
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
This is what I am getting off of pin 5 of J103. If it is a sensor why is it a sine wave? The good news is at least it is stable. When I use the recommended test point, I was not able to get a stable waveform. Now at least I can ge the RPM set.


upload_2019-4-28_16-46-51.png
 
BTW: If 294.41 Hz is the frequency of that wave form, it is not too far from the 290 RPM or is that unrelated? What happens to that signal when you adjust R142 as per the field service guide?

The correct tach frequency appears to depend on the pulley diameter and there are three different kinds used (7-10). It varies from 305-390, depending on pulley.

(back to the C64 for me, this Atari stuff is too weird)
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
BTW: If 294.41 Hz is the frequency of that wave form, it is not too far from the 290 RPM or is that unrelated? What happens to that signal when you adjust R142 as per the field service guide?

The correct tach frequency appears to depend on the pulley diameter and there are three different kinds used (7-10). It varies from 305-390, depending on pulley.

(back to the C64 for me, this Atari stuff is too weird)
Wow sounds like you know the manual better than me. ;)

If I vary R142 it changes the RPM.

There are actually a couple of versions of that manual. The one I was looking at does not have that chart and a completely different method to set the rpm.

From that chart, it looks like the one I should have is 305 RPM (I have the black belt. I tried to get an accurate measurement on the pulley, but it is nearly impossible to get the mircometer in there for an accurate measurement, though ti appears to be closer to .3 than .25"

The frequency is unstable. Varies by at least 3khz. Might replacing c143 help?

But what I really want to do is to replace that circuit and maybe the whole board.
 
Not sure...I started to read the manual(s) and then I remembered that I have been doing that all week with my C64 and plus, Naked and Afraid is coming on soon :)

I looked at:
https://ia800204.us.archive.org/32/items/Atari810DiskDriveFieldServiceManualRev1/Atari_810_Disk_Drive_Field_Service_Manual_Rev_1_text.pdf

and

http://www.atarimania.com/documents/Atari_810_Disk_Drive_Service_Manual_Rev_2.pdf

..and yeah, they are different...from what I saw, Rev 2 gives specific advice on what to replace when...looked better to me.
 
OK can you or someone help me here. How do I get RPM out of this? If it were a pulse I could figure it out. But this sine wave is throwing me.

Hint specs say the RPM of the drive is 288RPM.
I don't think that you get RPM per say, I interpret that you use the the table to identify the Hz that it should be set at...to get the 288 RPM...sounds a little weird and maybe I am misreading it, but that looks like what they are saying.

p7-10.jpg

Edit: You could verify it if you had one of the strobe light speedo things.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
I don't think that you get RPM per say, I interpret that you use the the table to identify the Hz that it should be set at...to get the 288 RPM...sounds a little weird and maybe I am misreading it, but that looks like what they are saying.

View attachment 176227

Edit: You could verify it if you had one of the strobe light speedo things.

Just did. See above. So what I need to know is how to translate that sine wave to RPM if I want to do a redesign. I need to understand what is going on.

I guess what I really should do is to build a jig to isolate the senor output to see if the sensor is causing the sine wave or the circuit somehow. I can't see how it could be the circuit though.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Just did. See above. So what I need to know is how to translate that sine wave to RPM if I want to do a redesign. I need to understand what is going on.

I guess what I really should do is to build a jig to isolate the senor output to see if the sensor is causing the sine wave or the circuit somehow. I can't see how it could be the circuit though.

Went for a walk. Helps to clear the head. I was missing part of the formula while on my walk. I realized I needed the size of the large disk. It is 2.5 inches. So the small one is .3 inches if I I am determining that correctly.
 
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